1. Field of the Invention
The present invention relates to a method and an arrangement, both for cleaning semiconductor wafers, and more particularly to a method and an arrangement for removing unwanted metal impurities and particles from the surfaces of semiconductor wafers.
2. Description of the Related Art
Pattern defects resulting from unwanted metal impurities or particles adhered to semiconductor wafers are the most prominent of the defectives of VLSI (Very-Large-Scale Integrated) circuits Since particles adhere to the wafers during the steps of the manufacturing process, such as sputtering, ion-implantation, and plasma CVD, the wafers must be alternately treated by combining different kinds of cleaning fluids and deionized water in accordance with the kind of the particles caused in respective steps Further, it has been performed to remove a natural oxide film from the wafer surface or to activate the wafer surface.
FIG. 5 shows a conventional cleaning arrangement In the FIGURE, 1 is a fluid reservoir for placing a process fluid containing desired chemicals, 2 is a pump for pumping the process fluid from the fluid reservoir 1, 3 is a filter, 4 is a measuring/mixing tank for measuring the pumped process fluid and deionized water supplied through a deionized water supply line 5 and mixing them to prepare a cleaning fluid, 6 is a cleaning vessel for treating semiconductor wafers, and 8 is a supply line for supplying deionized water for rinse, respectively.
In the cleaning method performed by the cleaning arrangement shown in FIG. 5, the fluid reservoir 1 filled with the process fluid is connected to the process fluid supply line which is located remote from the cleaning arrangement Next, the process fluid is supplied by the pump 2 to the measuring/mixing tank 4 through the filter 3 until a level sensor (not shown) connected to the tank 4 indicates that the surface of the process fluid in the tank 4 reaches a predetermined level. Similarly, the deionized water is supplied to the measuring/mixing tank 4 through deionized water supply line 5 until the surface of the fluid in the tank 4 reaches a predetermined level In the tank 4, the process fluid and the deionized water are mixed to prepare a cleaning fluid. One of the cleaning fluids is supplied to the cleaning vessel 6 in turn, thereby treating the semiconductor wafers therewith. Thereafter, the semiconductor wafers 7 are rinsed with the deionized water supplied from the line 8.
However, the conventional cleaning arrangement and the conventional cleaning method have the following problems:
(a) Since the process fluid is carried after the reservoir is filled therewith, unwanted metal impurities and particles are dissolved in the process fluid from the reservoir. Therefore, a high purity process fluid can not be obtained.
(b) Since the process fluid is an aqueous solution of chemicals, it is necessary to frequently exchange process fluid reservoirs with one another. Therefore, the impurity contamination may occur from the external atmosphere, depending upon the frequencies of exchanging the reservoirs. Further, much care is necessary to maintain the safety of working.
(c) Since it is necessary to locate the reservoirs at the position remote from the cleaning arrangement, the length of piping may inevitably become long and be complicated. Therefore, when the process fluid is handled and the reservoirs are frequently exchanged, the chance of leakage of the process fluid and the chance of intrusion of unwanted contaminants may be increased in the piping. Consequently, it may be difficult to prepare a high purity process fluid.
(d) Due to the facts (a) to (c), it is not easy to change the composition of the process fluid. Although the process fluid is measured by the level sensor, it is difficult to accurately control the composition of the process fluid due to the length of piping.
There has been proposed a cleaning technique such that after supplying deionized water and a cleaning gas to a cleaning vessel to provide a cleaning fluid, semiconductor wafers are cleaned with the cleaning fluid.
FIG. 6 shows such a method which is disclosed in Published Unexamined Japanese Patent Application No. 2-164035. In FIG. 6, 1 is a reservoir 1 for deionized water, 2 is a heater, 3 is an ozone generator, 4 is a pump, 5 is a filter, 6 is a cleaning vessel, 7 is a gas supply unit 7, and 8 is a filter.
Semiconductor wafers are cleaned by the method in the following manner.
The ozone introduced into the reservoir 1 is partly dissolved in the deionized water contained in the reservoir 1 to produce ozone-containing deionized water. The ozone-containing deionized water is supplied to the cleaning vessel 6 through the filter 5. Meanwhile, ammonia gas, hydrogen chloride gas, or nitrogen dioxide gas is supplied to the cleaning vessel 6 through the filter 8 from the gas supply unit 7. Using such a cleaning fluid, semiconductor wafers are cleaned, thereby preventing unwanted particles from adhering to the surfaces of the wafers, and preventing unwanted impurities from contaminating the surfaces of the wafers.
The cleaning method, however, has the following problems:
(a') When the ammonia gas is used as the cleaning gas, ammonia water, which acts as a cleaning agent, cannot be uniformly dissolved in the deionized water in the cleaning vessel 6. In other words, it is difficult to obtain a cleaning fluid having a predetermined concentration of the process fluid. Further, it is impossible to control the concentration of the process fluid in the cleaning fluid correctly.
(b') Semiconductor wafers cannot be continuously treated with cleaning fluids each containing a different process fluid.
(c') Even if the process fluid remained in the previous step is removed, it is difficult to maintain the deionized water clean in the cleaning vessel.
(d') An agitator or a bubbler needs to be used to dissolve ammonia water (i.e., cleaning agent) uniformly in the deionized water. It may be undesirable, however, that rotational members such as the agitator be located within the cleaning vessel.